The present invention relates to methods for making preforms for use in friction brakes.
Carbon brakes are conventionally manufactured by pressing preforms into required shapes. The preforms are made from a combination of structural forms, such as particles or powders, short fibers, and flakes. Some or all of the structural forms are carbon or carbon-based.
With conventional methods of manufacture, it has been difficult to maintain a uniform mixture or distribution of the structural forms throughout the preform. In general, the combination of different structural forms naturally tends to separate or xe2x80x9cdemix,xe2x80x9d especially when the structural forms are dry-blended.
One conventional solution to the demixing problem is to create a wet slurry of the structural forms. However, when the slurry is dewatered (using, for example, a screen), the particulate elements tend to be lost through the screen as the water drains. This leaves a non-uniform distribution of the structural forms. On the other hand, filtering the slurry will retain the fine particulate elements, but it results in increased processing times and non-uniform distribution of the structural forms.
Klett, U.S. Pat. No. 5,744,075, discloses a slurry-based method of making a densified carbon matrix carbon fiber composite preform which comprises the use of a rigidizerxe2x80x94such as paraffin wax, polyethylene glycol, and mixtures thereofxe2x80x94and a vacuum molding step.
Other conventional methods, such as spray drying, do not work well with a mixture containing fiber elements.
Flocculation is a known process by which fine particles, suspended in a liquid, form an aggregate or a clumped mass. A principal conventional use of flocculation is to aid in making solid-liquid separations, such as: removing suspended matter in drinking water; separating solid material in municipal or industrial wastewater to obtain low-volume, dewatered sludge; removing suspending inorganic material from waste streams; separating solid and liquid phases in leaching operations, where a valuable material is contained in the liquid phase, thereby facilitating recovery; and binding fine cellulose fibers and inorganic additives to long cellulose fibers as paper pulp is formed into thin sheets. See generally Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 11, 4th Ed., John Wiley and Sons, 1994, pp. 61-80, and especially, p. 62. See also Asami, U.S. Pat. No. 5,906,712, which discloses a flocculation process involving latex, for making paper-thin composite sheets.
It is desirable to provide a method for manufacturing a preform that encourages a uniform distribution of structural forms (for example, without limitation, particles, powders, short fibers, or flakes) throughout the preform so as to avoid demixing. The present invention therefore uses flocculation of the structural forms constituting the preform in order to prevent demixing.
A method according to the present invention includes forming a slurry using a material including a plurality of structural forms, including, for example and without limitation, fibers, particles, and flakes. Either a cationic material or an anionic material is initially dispersed in the slurry so as to coat the structural forms of the material. Thereafter, an anionic material or a cationic material (i.e., the opposite of the material used to coat the structural forms of the material) is dispersed into the slurry having the coated structural forms, thereby causing flocculation of the structural forms in the slurry in a known manner. Finally, the flocced slurry is dewatered, shaped, and dried to obtain a desired preform.
It should be noted that the present invention lies, most generally, in the use of the flocculation phenomenon to ensure a uniform distribution of structural forms in the preform.
As indicated above, the present invention relates to manufacturing a preform for friction elements that, in pertinent part, uses flocculation to provide a uniform distribution of the structural forms constituting the preform. Accordingly, the present invention is applicable to a wide variety of material combinations and structural form combinations.
Generally, the material or materials that are to be consolidated into a preform are slurried. Water is a suitable vehicle for making the slurry. Each material characteristically includes at least two different structural forms, including, for example and without limitation, particles of various sizes, fibers, and flakes in accordance with the respective material or materials used.
As mentioned above, one or more materials can be mixed to form the slurry. In one example, sintered carbon is used, comprising carbon powder particles and some short carbon fibers. In another example, carbon fibers, silicon carbide, and phenolic powder is mixed. The short fibers may be, for example, chopped or milled to form fibers of the appropriate length.
The cationic and anionic materials are, preferably (but not necessarily), cationic and anionic polymers. In one example in accordance with the present invention, a cationic polymer (such as that commercially available under the tradename xe2x80x9cKymene 557Hxe2x80x9d available from the Hercules Co.) is dispersed in the slurry of the material or materials to coat the structural forms in the slurry. Thereafter, an anionic polymer (such as that commercially available under the tradename xe2x80x9cReten 235xe2x80x9d from the Hercules Co.) is dispersed in the slurry containing the structural forms coated with cationic polymer.
The electric charge interaction between the cationic polymer coating the structural forms in the slurry and the anionic polymer subsequently dispersed in the slurry causes the structural forms to floc or clump together in a mass having a desirably uniform distribution of structural forms throughout. The flocced mass is then dewatered and shaped in a form. The dewatered mass is then dried to obtain the desired preform (which is ready for a subsequent pressing operation). If desired, the dewatering and shaping can be carried out simultaneously in a form having a screen.
The form is preferably shaped to provide a preform having a desired shape, such as a block or a right cylinder.
The flocculation phenomenon that occurs with the use of cationic and anionic materials in general occurs regardless of which is used first and which is used subsequently. However, specific cationic materials and anionic materials may have a unique behavior relative to each other such that one or the other must be used first relative to a counterpart material used second.
The amounts of cationic and anionic materials that are necessary to bring about flocculation depends on the amount of the constituent material or materials present. It is desirable to use the smallest effective amounts of the cationic and anionic materials so as to reduce the usage of those materials and thereby reduce processing costs.
A preferred method for making a preform in accordance with this invention comprises: forming a slurry including particulate materials having a plurality of structural forms, one of which is carbon fibers; dispersing a cationic material into the slurry so as to substantially coat the plurality of structural forms with the cationic material; dispersing an anionic material into the slurry having the coated structural forms therein so as to cause the coated structural forms to flocculate; and drying the flocced slurry in a form to obtain a preform. As illustrated in the Examples hereinbelow, the preform may conveniently have a thickness ranging from approximately xc2xe inch (0.78 inches) through 10 inches. In the preferred embodiments of the invention, fibers will constitute 1-90 weight-%, preferably 1-50 weight-%, of the particulate materials and powders will constitute 10-99 weight-%, preferably 50-99 weight-%, of the particulate materials. The amount of cationic material used will constitute 5-25 weight-%, based on the total weight of the particulate materials. The anionic material used will be constituted as a 0.05-1.0% solution, employed at a rate of 1 to 5 liters per kilogram of particulate material.